1. Field of the Invention
The present invention relates to a metal powder and a method for manufacturing the same. More particularly, the present invention relates to a method for manufacturing a metal powder for use in an electroconductive paste suitable for forming internal electrodes of a multilayer ceramic electronic component.
2. Description of the Related Art
Conventionally, a multilayer ceramic electronic component, for example, a multilayer ceramic capacitor, comprises a ceramic laminate, internal electrodes and external electrodes. The ceramic laminate is obtained, for example, by baking a raw ceramic laminate formed by laminating a plurality of raw (green) ceramic layers made of a dielectric material. The internal electrodes which are situated between the ceramic layers in the ceramic laminate are formed by providing printing with an electroconductive paste onto a plurality of raw ceramic layers followed by baking together with the raw ceramic laminate. One edge of each of the internal electrodes is exposed to one edge surface of the above-described ceramic layers. The external electrodes are formed by applying an electroconductive paste to the edge surfaces of the ceramic laminate and then baking so as to connect to the edges of the internal electrodes exposed at the edge surfaces of the ceramic laminate.
An electroconductive paste is used for such a multilayer ceramic electronic component, and especially for forming internal electrodes of a multilayer ceramic capacitor. The electroconductive paste contains a metal powder that functions as an electroconductive component. In recent years, a base metal powder such as nickel powder and copper powder has come to be used as the metal powder, in addition to a noble metal powder such as silver and palladium powders previously used. Furthermore, as progress has been made in realizing miniaturization and thickness reduction of the layers of a multilayer ceramic electronic component, finer particles are demanded for the metal powder contained in an electroconductive paste for use in forming such internal electrodes.
As the particle size of a metal powder becomes smaller, the specific surface area of the powder grows in proportion to the second power of the particle size, and as the specific surface area grows, the sintering temperature of the metal powder becomes lower. If internal electrodes of a multilayer ceramic electronic component are formed with an electroconductive paste comprising a metal powder with such a small particle size, sintering of the metal powder proceeds rapidly before sintering of the ceramic powder used to form the ceramic laminate begins, tending to cause structural defects such as delamination, owing to the difference of the shrinkage behaviors between the ceramic powder and the metal powder.
Japanese Unexamined Patent Application Publication 4-45835 discloses a method for causing local hydrolysis with a metal alkoxide on the surface of a metal particle so as to form compact layers of minute metal oxide particles such as titania or silica on the surface. Furthermore, Japanese Unexamined Patent Application Publication 11-45617 discloses a method for coating particles of a base metal such as manganese or manganese oxide. However, if an electroconductive paste comprising such a metal powder is used for forming an electrode film, and the film is baked at the same time with ceramic green sheets, the silica or the like coating the metal powder diffuses into the ceramic and accelerates the grain growth of the ceramic grains, with the result that such growth of ceramic grains may entail problems of interlayer delamination and crack generation.
Furthermore, Japanese Unexamined Patent Application Publication 10-324906 discloses a method for manufacturing a nickel powder by a spray thermal decomposition method in which, for example, composite oxides comprising lanthanum and nickel are deposited on the surface of the nickel powder. It discloses that contact between nickel metal particles can be prevented when internal electrodes are formed with an electroconductive paste comprising such a nickel powder, and therefore the sintering temperature of the nickel powder can be shifted to a higher level, with the result that a ceramic electronic component can be obtained which is prevented from delamination. However, sintering of the nickel according to this method proceeds rapidly once the coating film made of the composite oxides existing on the surface of the nickel powder is destroyed during the baking step, with the result that there appears a problem of rapid stress generation in the internal electrodes which will induce crack generation.
A metal powder gas phase manufacturing method such as the spray thermal decomposition method also has a problem in that it is more expensive and more complex than a liquid phase metal powder manufacturing method.
Accordingly, it is an object of the present invention to solve the above-described problems, and to provide a metal powder and a manufacturing method therefor wherein sintering of the metal powder is restricted at a low temperature, the sintering initiation temperature is shifted to a higher level, and rapid sintering shrinkage is restricted, while grain growth of a ceramic is not accelerated.
For achieving the above-described object, the metal powder manufacturing method according to the present invention is characterized in that a metal powder is obtained from a mixture comprising a caustic alkali, hydrazine and/or hydrazine hydrate, a salt of electroconductive metal, a rare earth metal salt and a main solvent, by depositing a rare earth metal hydroxide derived from the above-described rare earth metal salt, and by reducing the above-described salt of electroconductive metal.
In particular, the metal powder manufacturing method according to the present invention preferably comprises the steps of: preparing a reducing solution comprising the caustic alkali, the hydrazine and/or hydrazine hydrate and the main solvent; preparing a metal salt solution comprising the salt of electroconductive metal, the rare earth metal salt and the main solvent; and mixing the reducing solution with the metal salt solution to form a metal powder by depositing a hydroxide derived from the rare earth metal salt and by reducing the salt of electroconductive metal.
The present invention is characterized in that the reduction reaction of a salt of electroconductive metal and the deposition reaction of a rare earth metal hydroxide are performed simultaneously in one reaction step so that a metal powder is recovered in which the metal obtained by the reduction of the salt of electroconductive metal and the deposited rare earth metal hydroxide exist in an intermingled state. Such a metal powder is different from a metal powder in which minute metal oxide particles or the like coat only the surface of the metal in that a rare earth metal hydroxide is intermingled with the metal powder, with the result that not only are the metal particles formed by reducing the salt of electroconductive metal are prevented from contacting with each other but also undesirable migration of the metal particles can be prevented. Accordingly, rapid sintering of the metal powder can be prevented at the time of baking an electroconductive paste comprising this metal powder.
It is noted that the state in which a rare earth metal hydroxide is intermingled with a metal powder is a state in which minute rare earth metal hydroxide particles exist on the surface of as well as inside the metal powder thus obtained.